Electron Exchange-correlation Effects Research Articles

Quantum machine learning corrects classical forcefields: Stretching DNA base pairs in explicit solvent.

In order to improve the accuracy of molecular dynamics simulations, classical forcefields are supplemented with a kernel-based machine learning method trained on quantum-mechanical fragment energies. As an example application, a potential-energy surface is generalized for a small DNA duplex, taking into account explicit solvation and long-range electron exchange-correlation effects. A long-standing problem in molecular science is that experimental studies of the structural and thermodynamic behavior of DNA under tension are not well confirmed by simulation; study of the potential energy vs extension taking into account a novel correction shows that leading classical DNA models have excessive stiffness with respect to stretching. This discrepancy is found to be common across multiple forcefields. The quantum correction is in qualitative agreement with the experimental thermodynamics for larger DNA double helices, providing a candidate explanation for the general and long-standing discrepancy between single molecule stretching experiments and classical calculations of DNA stretching. The new dataset of quantum calculations should facilitate multiple types of nucleic acid simulation, and the associated Kernel Modified Molecular Dynamics method (KMMD) is applicable to biomolecular simulations in general. KMMD is made available as part of the AMBER22 simulation software.

Total, state-selective, and angular-differential cross sections for electron capture in He2++H collisions in warm dense plasmas

The total, state-selective, and angular-differential cross sections for He2++H collision system in warm dense plasmas are studied by using the two-center atomic orbital close-coupling method in the energy range 0.1–300 keV/u. The calculations are performed for plasma density and temperature ranges ne ∼1018 to ∼1021 cm−3, Te = 0.3 eV–1.2 eV, typical for the H- and He-rich white dwarfs. The plasma environments are described by a unified screened potential involving electron degeneracy, finite-temperature gradient, and exchange-correlation effects. The results for H++H cases with the same plasma parameters are also presented for comparison to elucidate the discrepancies of plasma screening effects on the electron capture dynamics for collision systems with different nuclear symmetries. Moreover, classical Debye screening results are also given for comparison to clarify the effects of quantum correlations in warm dense plasmas on the electron capture dynamics. The present work is expected to provide theoretical and data support for the astrophysical plasmas.

Ab-initio Study of Structural and Electronic Properties of Perovskite Nanocrystals of the CsSn[Br1−xIx]3 Family

In this study, by means of quantum-chemical calculations within the framework of density functional theory, we considered a number of structural and electronic properties of nanocrystals of the CsSn[Br1−xIx]3 (systems CsSnBr3, CsSnBr2I, CsSnBrI2 and CsSnI3) and discussed the effect of iodine concentration on the geometry and electronic properties of these materials. The exchange correlation effects of electrons were taken into account by the LDA, GGA and the modified Becke-Jones exchange correlation potential (mBJ). The results obtained in the framework of the DFT-mBJ and the Wien2k packages are in good agreement with the data from experimental measurements and open up the possibility of accurately predicting a number of fundamental properties of perovskite-like complex structures and the development of new materials. Doi: 10.28991/HIJ-2022-03-02-03 Full Text: PDF

Electron exchange-correlation effects on dispersion properties of electrostatic waves in degenerate quantum plasmas

Based on the quantum Magnetohydrodynamic (QMHD) model, the obliquely propagation of electrostatic waves in degenerate magnetized quantum plasmas with electron exchange-correlation effects are theoretically investigated. The modified linear dispersion relations of electrostatic waves are obtained and discussed in some specific cases. The analytical results clearly show that the dispersion properties of the high frequency electron waves (including the Langmuir wave and upper-hybrid wave) and the low frequency ion acoustic wave are modified by the quantum effects together with the electron exchange-correlation effects. The numerical results depict that the Langmuir wave and upper-hybrid wave can be unstable in the presence of the electron exchange-correlation effects, and it is also evidently indicated that the electron exchange-correlation effects can reduce the phase velocity of the waves, especially in the high wave number region. The corresponding results should be of relevance for identifying electrostatic fluctuations which transport in an inhomogeneous and magnetized quantum plasmas.

High performance wave packet molecular dynamics with density functional exchange-correlation term for non-ideal plasma simulations

We report on development of the wave packet molecular dynamics (WPMD) with density functional theory (DFT) simulation technique that we proposed earlier for nonideal plasma and warm dense matter simulations. The method is based on the WPMD where the electron exchange-correlation effects are taken into account using the DFT approach. It is aimed at studying simultaneous dynamics of electrons and ions in equilibrium and non-equilibrium conditions for a wide range of temperatures and densities. Compared to classical molecular dynamics and WPMD simulations the method of WPMD-DFT provides more accurate representation of quantum effects such as electron–ion coupling and electron degeneracy. At this stage of the method development we pay a special attention to the performance issues such as acceleration with graphical processing units, the choice of an optimal simulation box size with respect to the boundary conditions, the use of an adaptive mesh for calculation of the exchange-correlation energy, and implementation of our algorithm in the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The results for internal energy of equilibrium dense hydrogen plasma are presented for evaluation of the method.

High-frequency surface waves in quantum plasmas with electrons relativistic degenerate and exchange-correlation effects

The propagation characteristics of high-frequency surface waves are studied in spin-1/2 quantum plasmas by considering the electron relativistic degenerate and exchange-correlation effects. Using the quantum fluid equations of magnetoplasmas in the presence of the quantum Bohm potential, spin magnetization energy, relativistic degenerate pressure, and exchange-correlation effects, a generalized dispersion relation is derived. The analytical and numerical results show that the relativistic degenerate and exchange-correlation effects significantly modify the propagation properties of high-frequency surface waves. It is found that under the influence of exchange-correlation effects, the frequency spectrum of high-frequency surface waves will be down-shifted. It is also indicated that the dispersion curve shifts up with the increase of relativistic gamma factor. Furthermore, the phase speed of the high-frequency surface waves increases with increasing electron number density. The current research is helpful to understand the propagation of the high-frequency surface waves in quantum plasmas, such as those in dense astrophysical environment.

Nonlinear dynamics of ion-acoustic waves in quantum plasmas with exchange-correlation effects

Abstract Nonlinear properties of ion-acoustic waves (IAWs) are studied in electron-ion (EI) degenerate plasma with the electron exchange-correlation effects by using the quantum hydrodynamic (QHD) model. To investigate arbitrary amplitude IAWs, we have reduced the model equations into a system of ordinary differential equations using a traveling wave transformation. Computational investigations have been performed to examine the combined effect of Bohm potential and exchange-correlation potential significantly modifies the dynamics of IAWs by employing the concept of dynamical systems. The equilibrium points of the model are determined and its stability natures are analyzed. The phase portrait and Poincaré return map of the dynamical system are displayed numerically. Quasiperiodic as well as chaotic dynamics of the system are confirmed through the Poincaré return map diagrams.

Electron-exchange potential correction on dynamics of multidimensional ion acoustic waves in quantum plasmas

We describe the propagation of arbitrary amplitude ion acoustic waves with electron exchange-correlation effects for two-dimensional quantum plasmas by using the quantum hydrodynamic model. The evolution of nonlinear waves in such plasmas is described by deriving a pseudoenergy-balance like equation, involving a Sagdeev-type pseudopotential. The effects of the key plasma configuration parameters, viz., quantum diffraction, electron exchange-correlation, and the angle of propagation of the wave, on the periodic and solitonic characteristics are studied in detail by employing the concept of dynamical systems. Also, we extend our investigation by considering an external periodic perturbation in a modified pseudoforce model. It is found that the dynamics of nonlinear ion acoustic oscillations in quantum plasma support periodic and quasiperiodic behavior depending on the external pseudofrequency. The implications of our results may have relevance in various dense astrophysical environments as well as in laboratory plasmas.

Propagation of surface waves in semi-bounded quantum collisional plasmas with electron exchange-correlation effects

The propagation of surface waves in semi-bounded quantum collisional plasmas are investigated by taking into account the quantum Bohm potential, Fermi statistical pressure, electron exchange-correlation effects and collisional effects. The modified quantum hydrodynamic model in conjunction with Maxwell equations are used to obtain the new general dispersion relations of surface waves, and the dispersion relations are discussed in some special cases of interest. It is indicated that the wave frequency spectrum can be down-shifted due to the electron exchange-correlation effects. It is also shown that the growth rate of surface waves instability can be enhanced by increasing the collisional frequency, especially in the short wavelength region. The corresponding results can be helpful for identifying surface waves which transport in intense metallic plasmas.

Nonlinear excitation and stability analysis of ion-acoustic waves in a magnetized quantum plasma with exchange-correlation effects of degenerate electrons

The nonlinear ion-acoustic wave excitation and its stability analysis are investigated in a magnetized quantum plasma with exchange-correlation and Bohm diffraction effects of degenerate electrons in the model. Using reductive perturbation technique, the Zakharov-Kuznetsov (ZK) equation is derived for two dimensional propagation of ion-acoustic wave in a magnetized quantum plasma. It is found that the phase speed, amplitude and width of the nonlinear ion-acoustic wave structures are affected in the presence of exchange-correlation potential in the model. The stability analysis of the 2D ion-acoustic wave pulse is also presented. It is found that growth rate of the first and second order instabilities of 2D ion acoustic wave soliton is enhanced with the inclusion of exchange-correlation potential effect in the model.

Intensities and shifts of Lyman and Balmer lines of hydrogen-like ions in high density plasmas

The spectral line intensities and line shifts of Lyman and Balmer series for transitions up to n = 5 of hydrogen-like ion are studied in plasmas with densities and temperatures in the ranges ne ∼ 1018–1021 cm−3, Te = 0.3–1.2 eV respectively. The screened potential used to describe the interaction between charged particles includes the electron exchange-correlation and finite-temperature gradient effects and is valid for both weakly and strongly coupled plasmas. The dependencies of alpha, beta and gamma line shifts of Lyman and Balmer series on plasma density (for fixed temperature) and temperature (for fixed density) are investigated. The results for the Hα line shifts are compared with the available high-density experimental data.

THz plasma wave instability in field effect transistor with electron diffusion current density

The plasma wave instability in rectangle field effect transistors (FETs) is studied with electron diffusion current density by quantum hydrodynamic model in this paper. General dispersion relation including effects of electrical thermal motion, external friction associated with electron scattering effect, electron exchange-correlation contributions and quantum effects were obtained for rectangle FETs. The electron diffusion current density term is considered for further analysis in this paper. It is found that the quantum effects, the electron diffusion current density and electrical thermal motion enhance the radiation power and frequencies. But the electron exchange-correlation effects and the electron scattering effects reduce the radiation power and frequencies. Results showed that a transistor has advantages for the realization of practical terahertz sources.

Raman and Brillouin scattering instabilities of transverse electromagnetic waves in degenerate electron-ion plasmas

By applying the Maxwell and quantum hydrodynamic equations, we have studied the parametric instabilities of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) in an unmagnetized electron-ion quantum plasma. In this context, we have derived the nonlinear dispersion relations of the large-amplitude electromagnetic (EM) waves, the electrostatic electron plasma waves, and the ion-acoustic waves. The nonlinear evolution equations are then solved by utilizing the Fourier transform to obtain expressions for the three-wave decay and modulational instabilities with quantum corrections. It is found that the growth rate of the instabilities is a strong function of large-amplitude EM waves, and quantum effects due to Fermi pressure and quantum correlations stabilize both SRS and SBS instabilities. Expressions for the maximum growth rates attributed to SRS and SBS instabilities are also derived by ignoring the nonlinear correction shift on the frequency of EM waves. The relevance of nonlinear interaction of EM waves with a quantum dense astrophysical plasma is highlighted in the perspective of electron Fermi pressure and exchange-correlation effects, where the plasma density is high enough.

Impact of electron exchange-correlation on drift acoustic solitary waves

The impact of electron exchange-correlation term on the linear and nonlinear quantum ion (QIA) acoustic drift waves in a highly degenerate plasma is investigated. An analytical approach is employed to derive the differential equation which is later on turned into Sagdeev energy integral equation that can be utilized to get drift solitons under existence conditions. It is noted that phase speed/frequency of the linear drift quantum ion acoustic (QIA) waves increases with electron exchange-correlation effect, but the amplitude of the corresponding solitons decreases with inclusion of these effects. Present study is carried out with reference to highly dense plasma environments like fast ignition inertial confinement fusion and white dwarfs etc.

Electron capture and excitation processes in collisions of H+ with H in finite-temperature quantum plasmas

Electron capture and excitation processes in H+-H collisions imbedded in warm dense quantum plasmas are studied by the two-center atomic orbital close-coupling (TC-AOCC) method in the energy range 1–200 keV. The screened potential which is used to describe the interaction between charged particles incorporates the quantum electron degeneracy, exchange-correlation and finite-temperature gradient correlation effects. The total and state-selective electron capture and excitation cross-sections for this collision system with this potential are calculated in a wide range of plasma densities and temperatures. It is demonstrated that the screening of Coulomb interactions affects the collision dynamics and the magnitude and energy behavior of the excitation and electron capture cross-sections. The results at very low and high temperatures are also compared with those obtained for zero-temperature quantum plasmas and Debye plasmas, respectively.

Propagation of electrostatic surface waves in a thin degenerate plasma film with electron exchange–correlation effects

Propagation of an electrostatic surface wave in a thin degenerate Fermi plasma film in the presence of constant external magnetic field is studied here. Dispersion relations for the symmetric and anti-symmetric modes have been derived and studied quantitatively with the exchange–correlation effects. It has been studied that with the increase in the strength of magnetic field, phase velocity of the waves decreases. Also electron exchange–correlation effects significantly modify the behavior of the surface waves such as frequency of surface wave is found to be downshifted by these effects. Moreover it has been studied that the group velocity of the anti-symmetric mode is greater than the symmetric mode for the whole wave numbers; however, these modes merge into a single mode with the increase of the wave number.

Spectral properties of hydrogen-like ions in finite-temperature quantum plasmas

The oscillator strengths and radiative transition probabilities for the Lyman and Balmer series for transitions up to n = 5 of a hydrogen-like ion embedded in a finite-temperature quantum plasma are calculated in a wide range of plasma densities and temperatures for the first time. The plasma screened Coulomb interaction includes the quantum electron degeneracy, exchange-correlation, and finite-temperature gradient correction effects. The results for the oscillator strengths at temperatures 0.35 eV and 3.5 keV are compared with those for zero-temperature quantum plasmas and for Debye plasmas, respectively.

Two-dimensional modeling and analysis of a nanometer transistor as a THz emitter

In this paper, we report on the influences of quantum effects, electron exchange-correlation, Fermi velocity, gate to channel distance and viscosity on the plasma frequency and instability of the plasma waves in a nanometer transistor. By extending the analysis to two-dimensional case, allowing oblique wave propagation, including viscosity and departing from gradual channel approximation, we obtain a general analytical expression for dispersion relation, plasma frequency, and “increment.” We found that, while the plasma frequency decreases with the electron exchange-correlation effect, it increases with quantum effects and Fermi velocity. It is shown that the spectrums of plasma waves are discrete both in longitudinal and lateral (transverse) direction. We also express the total radiated power in terms of transistor parameters especially the lateral dimension. Viscosity which is inherently presented in the structure and cannot be neglected, dramatically decrease the emitted power and set a lower limit on the length of transistor. We show that a nanometer transistor with a long width (a long lateral dimension) has advantages for the realization of practical terahertz emitters.

Response to “Comment on ‘Surface waves on quantum plasma half-space with electron exchange-correlation effects’” [Phys. Plasmas 23, 034701 (2016)

The main differences between our work [H. Khalilpour, Phys. Plasmas 22, 122112 (2015)] and the work of Shahmansouri [Phys. Plasmas 22, 092106 (2015)] are summarized. Also, the validity of the pre-factor 1/9 in front of Bohm potential is discussed.

Comment on “Surface waves on quantum plasma half-space with electron exchange-correlation effects” [Phys. Plasmas 22, 122112 (2015)

More recently, Khalilpour in his paper [Phys. Plasmas 22, 122112 (2015)] has studied the dispersion relation for propagation of surface waves in a half-space quantum plasma using a hydrodynamic model including the electron exchange-correlation effect, which he calls the modified quantum hydrodynamic model. While this paper looks similar to the previously published one [M. Shahmansouri, Phys. Plasmas 22, 092106 (2015)] in structure, the only difference appears to be inclusion of the recently suggested kinetic factor of γ=1/9 for the quantum Bohm potential. In this comment, we would like to clarify the application of the mentioned factor in the standard quantum hydrodynamic model and stress out that the so-called modified quantum fluid model does not exist and the current extension of author is meaningless in the context of quantum hydrodynamics.